In recent years, there has been a tremendous growth in mobile data traffics. Furthermore, this trend is expected to continue in the near future. To cope with this growth, dense transmission points, e.g., UDNs, need to be deployed. In particular, an UDN having a number of small cells having similar cell ranges and highly overlapped coverages is advantageous in dramatically increasing the system capacity, together with other advantages. Despite a number of advantages, ultra-dense small cell deployment also poses many challenges. Owing to highly-overlapped cell coverage, handover candidate BSs have similar received signal strength (RSS) values measured at a UE. In addition, there are a large number of such candidate BSs. Frequent occurrence of handover thus arises. Since frequent occurrence of handover causes high overhead or call drop issues, a proper handover management system has to be developed in order to address these issues. It is desirable to select a preferred BS whenever a handover is required such that the total number of handover events over a given time duration can be kept minimal or reduced. Efforts for selecting the preferred BS in order to lower the number of handover events include the following techniques.
In CN1889770A, the preferred BS is determined to be a BS based on determining whether the UE is approaching or leaving this BS. Whether the UE is approaching the BS is determined by first-order differences of (a) a received signal quality such as a SNR or an error rate, and (b) a time delay of signal arrival. The preferred BS is determined to be the BS that is considered by checking the first-order differences with predefined thresholds. Only the BS with a positive first-order difference, which means the UE is currently approaching the BS instead of leaving the BS, is chosen as a preferred BS. However, there are some drawbacks. First, fine synchronization and signal demodulation for all BSs under consideration are required. Second, threshold configuration is case-specific, and is difficult to find a universal threshold. Third, the use of first-order differences only provides coarse grouping or classification of the BSs. The preferred BS that is identified may not be the most desirable one in minimizing the number of handover events.
In US2009303891, the preferred BS is determined by considering RSS values, first-order differences thereof and Doppler frequency as prioritizing/weighting factors. Nevertheless, more measurement and estimation effort is required for this technique. Because of using first-order differences only, this technique also suffers from the same drawback of the technique of CN1889770A in that only coarse grouping of the BSs under consideration results.
In U.S. Pat. No. 8,934,906B2, a method for determining whether a train is approaching or leaving a candidate BS is suggested for subsequently determining a preferred BS. The train is installed with a relay BS on board. Periodic measurements on a time of arrival, a frequency offset, a Doppler frequency, a RSS value, or a combination thereof, are performed by the relay BS. A preferred BS that the relay BS is advantageous to switch to is identified if the increasing rate of the signal quality revealed from the periodic measurements exceeds a threshold. There are several drawbacks regarding this technique, however. First, periodic measurements are not practicable for an ordinary UE such as a smartphone, which is often battery-power-limited. Second, threshold configuration may be case-specific and difficult to find a universal threshold. Third, this technique also inherits the drawback of other aforementioned techniques in that only coarse grouping of the BSs is resulted due to consideration of first-order differences only.
There is a need in the art to have an improved technique over existing techniques for selecting a preferred BS from a list of candidate BSs in order to lower the number of handover events without excessive implementation complexity.